Influenza viruses are one of the most ubiquitous viruses present in the world, affecting both humans and livestock. Influenza results in an economic burden, morbidity and even mortality, which are significant.
The influenza virus is an RNA enveloped virus with a particle size of about 125 nm in diameter. It consists basically of an internal nucleocapsid or core of ribonucleic acid (RNA) associated with nucleoprotein, surrounded by a viral envelope with a lipid bilayer structure and external glycoproteins. The inner layer of the viral envelope is composed predominantly of matrix proteins and the outer layer mostly of host-derived lipid material. Influenza virus comprises two surface antigens, glycoproteins neuraminidase (NA) and haemagglutinin (HA), which appear as spikes, 10 to 12 nm long, at the surface of the particles. It is these surface proteins, particularly the haemagglutinin that determine the antigenic specificity of the influenza subtypes. Virus strains are classified according to host species of origin, geographic site and year of isolation, serial number, and, for influenza A, by serological properties of subtypes of HA and NA. 16 HA subtypes (H1-H16) and nine NA subtypes (N1-N9) have been identified for influenza A viruses [Webster R G et al. Evolution and ecology of influenza A viruses. Microbiol. Rev. 1992; 56:152-179; Fouchier R A et al. Characterization of a Novel Influenza A Virus Hemagglutinin Subtype (H16) Obtained from Black-Headed Gulls. J. Virol. 2005; 79:2814-2822). Viruses of all HA and NA subtypes have been recovered from aquatic birds, but only three HA subtypes (H1, H2, and H3) and two NA subtypes (N1 and N2) have established stable lineages in the human population since 1918. Only one subtype of HA and one of NA are recognised for influenza B viruses.
Influenza A viruses evolve and undergo antigenic variability continuously [Wiley D, Skehel J. The structure and the function of the hemagglutinin membrane glycoprotein of influenza virus. Ann. Rev. Biochem. 1987; 56:365-394]. A lack of effective proofreading by the viral RNA polymerase leads to a high rate of transcription errors that can result in amino-acid substitutions in surface glycoproteins. This is termed “antigenic drift”. The segmented viral genome allows for a second type of antigenic variation. If two influenza viruses simultaneously infect a host cell, genetic reassortment, called “antigenic shift” may generate a novel virus with new surface or internal proteins. These antigenic changes, both ‘drifts’ and ‘shifts’ are unpredictable and may have a dramatic impact from an immunological point of view as they eventually lead to the emergence of new influenza strains and that enable the virus to escape the immune system causing the well known, almost annual, epidemics. Both of these genetic modifications have caused new viral variants responsible for pandemic in humans.
Influenza B virus antigenic drift is less frequent than that in the A strains and antigenic shift is unknown. Although antigenically distinct lineages (usually two, e.g. B/Yamagata and B/Victoria) of influenza B may occasionally co-circulate, with proportions varying from year to year and country to country, it is usual for influenza vaccines to contain only one influenza B strain.
HA is the most important antigen in defining the serological specificity of the different influenza strains. This 75-80 kD protein contains numerous antigenic determinants, several of which are in regions that undergo sequence changes in different strains (strain-specific determinants) and others in regions which are common to many HA molecules (common to determinants).
Influenza viruses cause epidemics almost every winter, with infection rates for type A or B virus as high as 40% over a six-week period. Influenza infection results in various disease states, from a sub-clinical infection through mild upper respiratory infection to a severe viral pneumonia. Typical influenza epidemics cause increases in incidence of pneumonia and lower respiratory disease as witnessed by increased rates of hospitalization or mortality. The severity of the disease is primarily determined by the age of the host, his immune status and the site of infection.
Elderly people, 65 years old and over, are especially vulnerable, accounting for 80-90% of all influenza-related deaths in developed countries. Individuals with underlying chronic diseases or with an impaired immune response are also most likely to experience such complications. Young infants also may suffer severe disease. These groups in particular therefore need to be protected. Besides these ‘at risk’-groups, the health authorities are also recommending to vaccinate health care providers.
Vaccination plays a critical role in controlling annual influenza epidemics. Currently available influenza vaccines are either inactivated or live attenuated influenza vaccine. Inactivated flu vaccines are composed of three possible forms of antigen preparation: inactivated whole virus, sub-virions where purified virus particles are disrupted with detergents or other reagents to solubilise the lipid envelope (so-called “split” vaccine) or purified HA and NA (subunit vaccine). These inactivated vaccines are given intramuscularly (i.m.), subcutaneously (s.c), or intranasally (i.n.).
Influenza vaccines for interpandemic use, of all kinds, are usually trivalent vaccines. They generally contain antigens derived from two influenza A virus strains and one influenza B strain. A standard 0.5 ml injectable dose in most cases contains (at least) 15 μg of haemagglutinin antigen component from each strain, as measured by single radial immunodiffusion (SRD) (J. M. Wood et al.: An improved single radial immunodiffusion technique for the assay of influenza haemagglutinin antigen: adaptation for potency determination of inactivated whole virus and subunit vaccines. J. Biol. Stand. 5 (1977) 237-247; J. M. Wood et al., International collaborative study of single radial diffusion and immunoelectrophoresis techniques for the assay of haemagglutinin antigen of influenza virus. J. Biol. Stand. 9 (1981) 317-330). Quadrivalent vaccines have also been reported, containing, in addition to the three classical strains, an additional B strain (Commun Dis Intell 2006, 30, 350-357) or an additional H3N2 strain (Vaccine 1992, 10, 506-511).
Interpandemic influenza virus strains to be incorporated into influenza vaccine each season are determined by the World Health Organisation in collaboration with national health authorities and vaccine manufacturers. Interpandemic Influenza vaccines currently available are considered safe in all age groups (De Donato et al. 1999, Vaccine, 17, 3094-3101).
Vaccine efficacy is however affected by the age and immune status of recipients and the match between vaccine and circulating influenza strains. There is little evidence that current influenza vaccines work in small children under two years of age. Furthermore, reported rates of vaccine efficacy for prevention of typical confirmed influenza illness are 23-72% for the elderly, which are significantly lower than the 60-90% efficacy rates reported for younger adults (Govaert, 1994, J. Am. Med. Assoc., 21, 166-1665; Gross, 1995, Ann Intern. Med. 123, 523-527). The effectiveness of an influenza vaccine has been shown to correlate with serum titres of hemagglutination inhibition (HI) antibodies to the viral strain, and several studies have found that older adults exhibit lower HI titres after influenza immunisation than do younger adults (Murasko, 2002, Experimental gerontology, 37, 427-439).
By way of background, during interpandemic periods, influenza viruses that circulate are related to those from the preceding epidemic. The viruses spread among people with varying levels of immunity from infections earlier in life. Such circulation, over a period of usually 2-3 years, promotes the selection of new strains that have changed enough to cause an epidemic again among the general population; this process is termed ‘antigenic drift’. ‘Drift variants’ may have different impacts in different communities, regions, countries or continents in any one year, although over several years their overall impact is often similar. Typical influenza epidemics cause increases in incidence of pneumonia and lower respiratory disease as witnessed by increased rates of hospitalisation or mortality. The elderly or those with underlying chronic diseases are most likely to experience such complications, but young infants also may suffer severe disease.
At unpredictable intervals, novel influenza viruses emerge with a key surface antigen, the haemagglutinin, of a totally different subtype from strains circulating the season before. Here, the resulting antigens can vary from 20% to 50% from the corresponding protein of strains that were previously circulating in humans. This phenomenon, called “antigenic shift” can result in virus escaping ‘herd immunity’ and establishing pandemics. In other words, an influenza pandemics occurs when a new influenza virus appears against which the human population has no immunity. It is thought that at least the past pandemics have occurred when an influenza virus from a different species, such as an avian or a porcine influenza virus, has crossed the species barrier. If such viruses have the potential to spread from human to human, they may spread worldwide within a few months to a year, resulting in a pandemic. For example, in 1957 (Asian Flu pandemic), viruses of the H2N2 subtype replaced H1N1 viruses that had been circulating in the human population since at least 1918 when the virus was first isolated. The H2 HA and N2 NA underwent antigenic drift between 1957 and 1968 until the HA was replaced in 1968 (Hong-Kong Flu pandemic) by the emergence of the H3N2 influenza subtype, after which the N2 NA continued to drift along with the H3 HA (Nakajima et al., 1991, Epidemiol. Infect. 106, 383-395).
Several clinical studies have been performed to evaluate safety and immunogenicity in unprimed populations, with monovalent candidate vaccines containing a pandemic strain such as the non-circulating H2N2 or H9N2 strains. Studies have investigated split or whole virus formulations of various HA concentrations (1.9, 3.8, 7.5 or 15 μg HA per dose), with or without alum adjuvantation. Influenza viruses of the H2N2 subtype circulated from 1957 until 1968 when they were replaced by H3N2 strains during the ‘Hong Kong pandemic’. Today, individuals that were born after 1968 are immunologically naïve to H2N2 strains. These vaccine candidates have been shown to be immunogenic and well tolerated. Results are reported in Hehme, N et al. 2002. Med. Microbiol. Immunol. 191, 203-208; in Hehme N. et al. 2004, Virus Research 103, 163-171; and two studies were reported with H5N1 (Bresson J L et al. The Lancet. 2006: 367 (9523):1657-1664; Treanor J J et al. N Engl J Med. 2006; 354:1343-1351). Other studies have reported results with MF59 adjuvanted influenza vaccines. One study has reported that two doses of an H5N3 influenza vaccine adjuvanted with MF59 was boosting immunity to influenza H5N1 in a primed population (Stephenson et al., Vaccine 2003, 21, 1687-1693) and another study has reported cross-reactive antibody responses to H5N1 viruses obtained after three doses of MF59-adjuvanted influenza H5N3 vaccine (Stephenson et al., J. Infect. Diseases 2005, 191, 1210-1215).
Persons at risk in case of an influenza pandemic may be different from the defined risk-groups for complications due to seasonal influenza. According to the WHO, 50% of the human cases caused by the avian influenza strain H5N1 occurred in people below 20 years of age, 90% occurred among those aged <40. (WHO, weekly epidemiological record, 30 Jun. 2006).
During a pandemic, antiviral drugs may not be sufficient or effective to cover the needs and the number of individuals at risk of influenza will be greater than in interpandemic periods, therefore the development of a suitable vaccine with the potential to be produced in large amounts and with efficient distribution and administration potential is essential. One way would be to free rapidly some production capacity for the production of pandemic influenza vaccine. However since the mass vaccination would only start after the onset of the pandemic, this would anyway result in important delays caused by the time needed to identify the strain and to produce the first lot. Moreover there will be a fixed and therefore limited weekly output by the vaccines manufacturers. This means that the current approach will almost certainly leave large parts of the population, globally unprotected when the pandemic strikes. The vaccine would be available too late for large parts of the population and mortality is anticipated to be high with an estimated 180-360 million deaths globally.
One way to address this current dilemma is to generate a pandemic vaccine in advance of a pandemic, and to design a monovalent vaccine with the “best candidate pandemic strain” instead of trivalent vaccines in an attempt to reduce vaccine volume, primarily as two doses of vaccine may be necessary in order to achieve protective antibody levels in immunologically naïve recipients (Wood J M et al. Med Mircobiol Immunol. 2002; 191:197-201. Wood J M et al. Philos Trans R Soc Lond B Biol Sci. 2001; 356:1953-1960). This vaccine could then be used for stockpiling or for priming of the population during the interpandemic period.
Unfortunately since current production facilities are entirely consumed year-round with the production of seasonal trivalent vaccines for annual vaccination in the Northern and Southern hemispheres, both approaches mentioned above are not feasible because there is no additional production capacity to produce that “best candidate pandemic strain”. In addition, it should be noted that there is an urgent need to find a solution to cope with shortages of interpandemic influenza vaccines which are regularly encountered during the annual influenza seasons. One solution could be to build additional production capacities, this would however require several years of construction, which is anyway an inadequate approach should a pandemic strike within the next years.
At present, the sole way to rapidly gain additional capacity would be to shorten the production time for the annual trivalent vaccine. Recurrent shortages of influenza vaccines occur each season in most of the countries which precludes an optimal coverage of the population regarded as high risk for developing severe influenza disease and complications. As antigen production is the rate-limiting factor in influenza vaccines production, alternative antigen sparing strategies have to be explored.
Some approaches rely on adjuvantation, the aim of which is to increase immunogenicity of the vaccine in order to be able to decrease the antigen content (antigen sparing) and thus increase the number of vaccine doses available. The use of an adjuvant may also overcome the potential weak immunogenicity of the antigen in a naïve population. Several approaches have been published. Examples have been shown using whole inactivated H2N2 or H9N2 virus adjuvanted with aluminium salt (N. Hehme et al. Virus Research 2004, 103, 163-171) or using a plain subvirion H5N1 vaccine or aluminium hydroxide adjuvanted split virus H5N1 vaccine (Bresson J L et al. The Lancet. 2006:367 (9523):1657-1664; Treanor J J et al. N Engl J Med. 2006; 354:1343-1351). The results of this last trial indicate that both plain and adjuvanted H5N1 virus vaccines are safe up to an antigen dose of 90 μg (tested only as plain subvirion vaccine). Using such a high dose of antigen is however not compatible with an antigen-sparing strategy rendered essential in the case of a pandemic. A sub-unit influenza vaccine adjuvanted with the adjuvant MF59, in the form of an oil-in-water emulsion is commercially available for the elderly and at risk population, and has demonstrated its ability to induce a higher antibody titer than that obtained with the non-adjuvanted sub-unit vaccine (De Donato et al. 1999, Vaccine, 17, 3094-3101). However, in a later publication, the same vaccine has not demonstrated its improved profile compared to a non-adjuvanted split vaccine (Puig-Barbera et al., 2004, Vaccine 23, 283-289).
New vaccines which are effective whilst addressing the antigen-sparing considerations are therefore still needed. These new vaccines will have an acceptable if not improved immunogenicity, in particular against weakly or non-immunogenic pandemic strains or for the immuno-compromised individuals such as the elderly population. These new vaccines will also ideally have a cross-protection potential, such that they could be used as pre-pandemic or stockpiling vaccines to prime an immunologically naive population against a pandemic strain before or upon declaration of a pandemic.